- Created by: Benny52
- Created on: 07-01-19 19:46
- Eukaryotic - complex & include all animal & plant cells. Prokaryotic - smaller & simpler, e.g. bacteria.
- Eukaryotes - organisms made of eukaryotic cells. Prokaryotes - a prokaryotic cell (a single celled organism)
- Animal cells:
- Nucleus - contains genetic material that controls all cellular activity.
- Cytoplasm - where most of chemical reactions happen. Contains enzymes that control reactions.
- Cell membrane - holds cell together & controls ins and outs.
- Mitochondria - where most of aerobic respiration reactions happen.
- Ribosomes - where proteins are made.
- Plant cells - all ove above plus:
- Cell wall - Supports cell & strengthens it.
- Permanent vacuole - contains cell sap.
- Chloroplasts - where photosynthesis occurs. Contain green substance - chlorophyll - absorbs light needed for photosynthesis.
- Bacteria Cell: Cytoplasm, Cell membrane, Cell wall. Don't have 'true' nucleus - have single circular DNA strand floating freely in cytoplasm. May also contain 1 or more small rings of DNA - plasmids.
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- Light microscopes use light & lenses to form image of specimen & magnify it. Let us see individual cells & large subcellular structures - nuclei.
- Electron microscopes use electrons. Higher magnification than light microscopes. Also higher resolution - sharper image. Lets us see much smaller things in more detail - internal structure of mitochondria & chloroplasts. Lets us see tinier things - ribosomes, plasmids.
- Magnification of image = image size/real size
- Convert from micrometres (um) to mm - divide by 1000.
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More on Microscopy - Practical
- How to prepare slide to view onion cells.
- Add drop of water to middle of clean slide.
- Cut onion and separate into layers. Use tweezers to peel off some epidermal tissue from bottom of one of layers.
- Place tissue into water on slide using tweezers.
- Add drop of iodine solution - stain - highlights objects in cell by adding colour.
- Place cover slip on top. Don't get air bubbles under - obstruct view of specimen.
- Light microscope:
- Clip slide you've prepared onto stage.
- Select lowest-powered objective lens.
- Use coarse adjustment knob to move stage up to just below onjective lens.
- Look down eyepiece. Use coarse knob to move stage downwards until image is roughly in focus.
- Adjust focus with fine adjustment knob until you get clear image.
- If you need to see slide with greater magnification, swap to higher-powered objective lens and refocus.
- Draw observations with pencil with sharp point. Ensure it takes up at least half of page and it's drawn with clear, unbroken lines. No colouring or shading. If drawing cells, subcellular structures should be drawn in proportion. Include title and magnification it was observed under. Label important features.
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Cell Differentiation & Specialisation
- Differentiation - when cell changes to become specialised for its job. As cells change - develop different subcellular structures and turn into different types of cells. Can carry out specific fucntions. Most diiferentiation occurs as organism develops. Most animal cells - ability lost at early stage once they're specialised. Most plant cells don't lose ability. Cells that differentiate in mature animals mainly used for repaining and replacing cells. Some cells undifferentiated - stem cells.
- Sperm cells specialised for reproduction. Function is to get male DNA to female DNA. Long tail & streamlined head to help it swim. Lots of mitochondria in cell to provide energy needed. Carries enzymes in head to digest through egg cell membrane.
- Nerve cells specialised for rapid signalling. Function is to carry electrical signals round body. Long to cover more distance and have branched connections at ends to connect to other nerve cells and form network.
- Muscle cells specialised for contraction. Function is to contract quickly. Long so have space to contract and contain lots of mitochondria to generate energy needed for contraction.
- Root hair cells specialised for absorbing water and minerals. Are cells on surface of plant roots, which grow into long 'hairs' that stick out into soil. Gives plant big SA for absorbing water & mineral ions from soil.
- Phloem & Xylem cells specialised for transporting substances. Form phloem & xylem tubes - transport substances like food and water around plants. Xylem cells hollow in centre and phloem cells have very few subcellular structures, so stuff can flow through.
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Chromosomes & Mitosis
- Nucleus contains genetic material - chromosomes - coiled up lengths of DNA molecules. Each carries large no. of genes. Body cells - two copies of each chromosome normally - one from mother and one from father. Humans - 23 pairs.
- Body cells in multicellular organisms divide to produce new cells in cell cycle. Stage where cell divides - mitosis. Mutlicellular organisms use to grow or replace damaged cells. End of cycle results in 2 new cells identical to original, with same no. of chromosomes. Important stages of cell cycle:
- Growth & DNA replication: In a cell not dividing, DNA is spread out in long strings. Before division, cell has to grow & increase amount of subcellular structures. It then duplicates its DNA - one copy for each new cell. DNA copied and forms X-shaped chromosomes, each arm being an exact duplicate of the other.
- Mitosis: Chromosomes line up at centre of cell and cell fibres pull them apart. The two arms go to opposite ends of cell. Membranes form around each of sets of chromosomes - become nuclei of two new cells - nucleus has divided. Lastly, cytoplasm & cell membrane divide. Cell has now produced 2 daughter cells. Same DNA - identical.
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- Can divide to produce more stem cells. Found in early human embryos. Adults have, but only found in certain places like bone marrow. Unlike embryonic stem cells, they can't turn into any cell type at all, only certain ones like blood cells.
- Can be grown in lab to produce clones & made to differentiate into specialised cells - medicine or research.
- Medicine use adult stem cells to cure disease. Embryonic stem cells could be used to replace faulty cells in sick people - insulin-producing cells for diabetics, nerve cells for paralysed people by spinal injuries, etc. Therapeutic cloning - embryo could be made to have same genetic info as patient - stem cells produced would also contain same genes so wouldn't be rejected by patients body if used to replace faulty cells. Risks involving using stem cells in medicine, e.g. stem cells grown in lab may become contaminated by virus which can be passed on to patient.
- Some people against stem cell research - feel human embryos shouldn't be used for experiments since each one is a potential human life. Others think curing existing suffering patients more important than rights of embryos. Embryos used in research are usually unwanted, which would've been destroyed.
- Plants - stem cells found in the meristems - parts of plant where growth occurs. Throughout plant's life, cells in meristem tissues can differentiate into any type of plant cell. Can be used to produce clones of whole plants quickly & cheaply - done for rare species as well. Also used to grow crops of identical plants with desired features.
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Diffusion & Osmosis
- The spreading out of particles from area of higher concentration to lower concentration. Happens in both solutions & gases - particles free to move about.
- Bigger the concentration gradient, the faster the diffusion rate. Higher temp - faster diffusion rate - particles have more energy so move round faster.
- Cell membranes - hold cell together but let stuff in and out too. Dissolved substances move in & out of cells by diffusion. Only very small molecules can diffuse through cell membranes.
- The larger the SA of the membrane, the faster the diffusion rate as more particles can pass through at once.
- Osmosis - The movement of water molecules across partially permeable membrane from region of higher water concentration to lower.
- Partially permeable membrane - has very small holes in it - only timy molecules can pass
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- Cells on plant roots grow into 'hairs' which stick out into soil. Each branch of root is covered in millions of these microscopic hairs. Gives plant large SA for absorbing water & mineral ions from soil. Mineral ions needed for healthy growth. Concentration of minerals usually higher in root hair cells than in soil around them, so root hairs take in minerals using active transport. Alllows plant to absorb minerals from very dilute solution, against concentration gradient. Also happens in humans - taking in glucose from gut and kidney tubules.
- Used in gut when there's lower concentration of nutrients in gut, but higher in blood. Glucose can be taken into bloodstream when its concentration in blood is higher than in gut.
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- Cells use diffusion to take in subatances needed and to rid of waste products. How easy it is for an organism to exchange substances with its environment depends on organisms SA:V ratio. The larger the organism, the smaller the SA compared to its volume. Shown by calculating SA:V ratios.
- Single-celled organisms - gases & dissolved substances can diffuse directly into or out of cell across cell membrane as they have large SA compared to their volume so enough substances can be exchanged across the membrane to supply the volume of cell.
- Multicellular organisms - smaller SA comared to volume - not enough substances can diffuse from their outside surface to supply their entire volume. Need some sort of exchange surface for efficient diffusion. Are adapted to maximise effectiveness: Thin membrane so substances only have short distance to diffuse. Large SA so lots of substance can diffuse at once. Exchange surfaces in animals have lots of blood vessels, to get stuff into & out of blood quickly. Gas exchange surfaces in animals often ventilated - air moves in & out.
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- Gas exchange happens in lungs. Job of lungs - transfer O2 to blood and remove waste CO2 from it. To do this, lungs contain millions of little air sacs - alveoli - where gas exchange occurs. Alveoli specialised to maximise diffusion of O2 and CO2. They have: Enormous surface area, moist lining for dissolving gases, very thin walls and a good blood supply.
- Inside of small intestine covered in millions of villi. Increase SA in big way so digested food is absorbed quicker into blood. They have: single layer of surface cells, and a very good blood supply to assist quick absorption.
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More on Exchanging Substances
- CO2 diffuses into air spaces within leaves, then diffuses into the cells where photosynthesis occurs. Leaf's structure adapted so this can happen easily.
- Underneath of leaf - exchange surface. Covered in stomata which CO2 diffuses in through. Oxygen & water vapour diffuse out through stomata. Size of stomata controlled by guard cells. Close stomata if plant is losing water faster than being replaced by roots. Plants would soon wilt without them. Flattened shape of leaf increases area of exchange surface so it's more effective.
- Walls of cells in leaf - exchange surface. Air spaces in leaf increase area of surface so there's more chance for CO2 to get into cells.
- Water vapour evaporates from cells inside leaf. Escapes by diffusion as there's lots of it inside the leaf and less of it in air outside.
- Gills - gas exchange surface in fish. Water enters fish through mouth and passes out through gills. Oxygen diffuses from water into blood in gills & CO2 diffuses from blood into water. Each gill made of thin plates - gill filaments - give big surface area for exchange of gases. Gill filaments covered in lamellae - increases SA more. Have lots of blood capillaries to speed up diffusion. Also have thin surface layer of cells to minimise distance gases have to diffuse. Blood flows through lamellae in one direction and water flows over in opposite - maintains large concentration gradient between water & blood. Concentration of oxygen in water is always higher than in blood - increases diffusion rate of O2.
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